Jillian's Guide to Black Holes: Forming - Types - Outside - Inside - Finding - References - Websites

Black holes by Induction

    Question: How do you go looking for something that is by nature impossible to see?
    Answer: By not looking for it!
From all that I have read or seen when astronomers say something is suspected to be a black hole, it is not because they went looking for extreme gravitational fields. They label it 'black hole' because that's the only thing it could possibly be --- not star, not dead star, not a combination of stars. In Holmesian fashion astronomers eliminate the impossible, and whatever's left, however improbable, is the solution. I gather that for a long time black holes were considered only in the theoretical sense, as an astrophysicist's mental play-thing, instead of something that could actually exist (especially so frequently). When odd things turn up, such as supermassive yet very tiny brightly glowing things, black holes rear their invisible heads as one of the most viable explanations.

Why would someone shy away from saying an object is a black hole? Well, I admit that black holes are rather freaky. If the event horizon isn't bad enough, the singularity is. Physicists aren't really sure what happens near the singularity, let alone in it. Having an object with such questionable parts be the cure-all answer to a tricky problem feels almost...like an ad hoc explanation. Perhaps it reminds astronomers of the days when they believed the sun went around the earth. If this caution is a reaction to being burned by false assumptions, I can't fault it.

What clues do we have for finding black holes?

The Case of the X-ray Lighthouse

As I explained in the Outside section, material mucking about around the black hole forms a disk-like structure called an accretion disk. This disk also has twin jets of streaming high-powered particles that are perpendicular to this disk. Those jets make black holes powerful x-ray emitters. As it happens, there are these astronomers that scan the skies with x-ray telescopes. When such x-ray astronomers find a strong source of x-rays and trace it back, they sometimes find a very tiny source. Ah, could this be a black hole? Heh heh.

The Case of the Redshifted Object

Astronomers can locate black holes using their accretion disk in another fashion: redshifting. What's that? Well, it's similar to the Doppler effect. You know, when you're watching a car on the street come towards you and it's honking its horn, the horn seems to be higher-pitched than normal. When the car passes in front of you, the pitch seems okay. When the car moves away from you, the horn seems lower-pitched than normal. That's the Doppler effect.

Graph of the rotational speed along an accretion disk.1

Redshifting is very similar to that, except it deals with light. The light from an object that is moving away from you seems to have a lower frequency. What? A yellowish star that is moving away from you seems to be reddish. An x-ray source that is moving away from you very quickly could get redshifted enough to seem like a visible light source! The opposite of redshifting is blueshifting, which happens when an object is moving towards you. It's kinda like redshifting in reverse: objects that are reddish appear yellowish, and infrared objects could get blueshifted until they appear to put out visible light.

Astronomers can measure the redshift of an object and then calculate how fast it must be moving. By measuring the redshift of one side of a disk and the blueshift of the other side, they can get an idea of how fast the disk is rotating. If they know how fast the disk is rotating, and they can kinda estimate how large the disk is (which in itself is quite a tricky and complicated task), they can kinda guess the mass of the thing about which the disk rotates. If perchance some accretion disk should be going very fast around something that has at least 3 solar masses, ohmy, could that be a black hole? Heh heh.

The image to the right shows the rotational speed of some dust in the center of the galaxy M84. According to a Hubble Space Telescope's Space Telescope Imaging Spectrograph (STIS) press release, "...the change in wavelength records whether an object is moving toward or away from the observer. The larger the excursion from the centerline -- as seen as a green and yellow picture element (pixels) along the center strip, the greater the rotational velocity. This motion allowed astronomers to calculate that the black hole contains at least 300 million solar masses."1

The Case of the Quaking Quasars!


A quasar.2
Astronomers also use this technique to learn more about a curious class of objects called quasars. The term stands for quasi-stellar object. See, far away in the oldest parts of the universe are these extremely bright objects that look just like stars. They would have to be mind-bogglingly bright to be that visible at such large distances---they would have to be about 10 million solar masses. That's nice, but what does it have to do with black holes? These quasars have another peculiar quality: they fluctuate irregularly. The diameter of the quasar changes very quickly. Is this important? Yes; according to astronomers, for a quasar to be that bright, it would also have to be huge. We're talking mega-super-duper-massive stars. The only catch is that quasars fluctuate so quickly that, if it were a big ol' star, the matter would have to go faster than the speed of light, which matter can't do.

Deduction? Quasars must be small. They can't be stars. Oh dear, what could they be? Hmm, black holes with accretion disks were calculated to put out tremendous amounts of energy. Could quasars be black holes? Heh heh. Here's your run-of-the-mill quasar. It doesn't look like much, does it? Just some yellowy blob of a picture. That yellowy blob is 1.5 billion light years from earth, and it's designation is PHL 909. Actually, the blob is an elliptical galaxy (a very old, round galaxy) that's host to the quasar. There is an interesting article on the Hubble Space Telescope site regarding the idea of quasars being black holes: STSci-1996-35.

The Case of the Wobbling Star

A common term in astronomy is a binary system, a pair of stars that move around one another in a complicated orbit. Sometimes, astronomers find a star that behaves like it is part of a binary system, but they can't seem to find its partner. How do they find only one partner of a binary system? The visible binary member would appear to wobble. Astronomers are interested in stars that move for their own reasons, so they would track this wobble. It would be noticed that the star is following an orbit around another star, just like a binary system. The only problem is that there is no visible partner.

That in itself is not enough to say that the partner is a black hole. Many binary systems involve neutron stars. How? Well, there are three scenarios. When binary stars orbit one another, sometimes they get close enough that they steal each other's atmosphere. It happens readily enough. Eventually, one of the stars supernovas, leaving a white dwarf, a neutron star, or a black hole. That's the first scenario. What can also happen is that one of the stars reaches the end of its lifetime, and it supernovas. The remnant of that explosion could become a black hole, if it has sufficient mass. Situation three is a combination of the two other scenarios. A star in a binary system could steal atmosphere from its partner and supernova, not forming a black hole. The force of its explosion could make its partner supernova, as well, and it does form a black hole. Whew!


A binary system with a black hole.

So, how do scientists tell whether the invisible partner is a white dwarf, a neutron star, or a black hole? They calculate its mass by watching the movements of the partner. If the visible star holds a loose orbit, perhaps the partner is only a white dwarf. However, if the partner orbits tightly, it could be moving around a black hole. Say the mass is measured at greater than 3 solar masses. Could this be a binary system with a black hole? Heh heh.

The Case of the Galactic Turning-Point


Optical and ultraviolet view of a galaxy. 3
So far all I've mentioned are the tiny black holes and not a peep did I give about the more easily found super-massive black holes. How forgetful of me! I hope you're familiar with the structure of a galaxy: the core and some stuff surrounding it. That stuff could be spread out in a spiral shape or close to the core in a spherical shape, among other things. It's the core that's interesting. Y'see, the cores of some galaxies are quite actively shooting out particles at high speeds in these jets that look suspiciously familiar to the jets on an accretion disk. Look at the picture on the left

Normal galaxy, powerful jets. Astronomers, curious as they were, peeked at the cores of these galaxies. They found the stars near the core were revolving about something rather quickly. Judging from their speeds, that something was about as massive as a couple million suns---and yet, where was it?

Even more curious, astronomers peeked at the cores of galaxies that weren't streaming energy, and found pretty much the same thing! It looked like most galaxies had these very small extremely massive things at their very cores. They collected some data, developed some theories, and found that the mass of the invisible thing in the center was usually directly proportional to the galaxy. A large galaxy would have a large center turning-point mass, and a small galaxy a small turning-point mass. This also links in with the quasars, for the mass at the center of the galaxies was capable of supplying the energy required to fuel a quasar---and in some cases it was a quasar! Something that large and that small at the center of a galaxy could only be a super-massive black hole, right? Heh heh.

The Case of the Gravitational Wave

Gravitational waves are tricky because I only recently learned what they are. What are they? In short and horribly simplified large accelerating bodies like binary star systems radiate energy in the form of gravitational waves as the binary members lose gravitational potential energy. Translation: as the stars in a binary get closer to one another, they give off gravitational waves. These waves are ripples of spacetime itself (unlike light rays, which travel through spacetime). They propagate at the speed of light and are not altered by the matter through which they pass. They affect the stuff they pass through by alternately compressing and rarifying things. Yeech, that didn't make much sense. Go to the Guide to Gravitational Waves for a much better description, okay?

Anyway, if detected, these waves could pinpoint binary pairs of black holes and possibly even singular black holes (that's almost a pun --- get it? singular? singularity? ... laugh, darnit!). These guys would be unique and identifiable by their signatures and would give a wealth of information about their current status. Binaries are more fun because they're slightly easier for the earth-based detectors to find and because they're not just sitting still like lumps. Binaries do fun stuff like rotate and inspiral. The detection of a gravitational wave would verify once 'n' for all and without a doubt the existence of black holes, and there are no few people currently developing detectors of all sorts.

I should explain the table a bit. In the Galaxy column you'll find the name of the host galaxy, the place where the black hole is. The NGC/M designations are tricky because there are two different classification systems for things that are not stars. The 'M-number' designation is the object's Messier number, and the 'NGC-number' designation is the object's New General Catalog number. Some objects belong to both systems. The Hubble Article Available column links to any press releases on the Hubble Space Telescope site from which the information came; some objects, such as M87, have multiple articles from earliest to latest. The Chandra Article Available column links to any press releases on the Chandra X-Ray Observatory site. The Type column tells you what kind of galaxy the object is, from Spiral to Elliptical to Irregular. As I recall from astronomy class, the classification of galaxies is a little less than an exact science. It helps ever so much that we only get one perspective of each galaxy. Distance column gives the distance in light years to the object. Mass refers to the mass of the black hole, not the host galaxy and is given in the units of solar masses. This table is roughly up-to-date as of October, 2003.

Galaxy
Chandra
Article Available
Hubble
Article Available
Type
 Distance1 Mass2 of Core Black Hole
0313-192 Galaxy has radio jets unusual for a spiral; joint VLA, Hubble, and Gemini-South results  
Radio
900 million
  
3C66B Radio galaxy has unusual "braided" optical jet  
 
270 Million
  
3C273 Hubble's new ACS camera discerned a spiral plume and dust lane previously obscured by the brightness of the quasar  
Quasar
1.9 billion
  
3C368, 3C324 and 3C265 Radio galaxies  
   
1 Billion
4C41.17 Extraordinarily distant radio galaxy with knotty core  
 
10 Billion
  
Circinus, the Compass, is an Active Galactic Nucleus  
Spiral
    
Markarian 315 is an Active Galactic Nucleus with fainter nucleus 6,000 light-years from cental nucleus; 240,000-light-year long jet-like feature between the nuclei  
Spiral
    
Milky Way: Sagittarius A*, Exhibited violent, rapid x-ray flare
,
 
Sbc
28,000
2.6 Million
Milky Way: Cygnus XR-1 black hole in a binary system; interesting pulses of UV light measured, possibly as material fell and cross event horizon; interesting evidence for rotating versus static black hole
Stellar
6,000
  
Milky Way: GRO J1655-40, Black hole x-ray binary moving at 250,000 miles per hour, thought to have been propelled by supernova blast  
Stellar
3,200 - 9,600
  
Milky Way: XTE J1118+480 Black hole in a binary system with sun-like star moving 300,000 mph (145 km/s) in eccentric orbit around Milky Way
Stellar
5,000
7
Milky Way: XTE J1550-564, X-ray jets shown emitting particles at 1/2 speed of light and then slowing down
 
Stellar
17,000
  
Milky Way: SS433 Stellar binary detected by measuring redshift of iron atoms in two lobes of gas .25 light years from black hole
 
Stellar
16,000
  
Milky Way: NGC 7078 = M15, Globular cluster in Milky Way, intermediate mass black holes; reinforcing the idea of the black hole being proportional to the environment  
Globular Cluster
32,600
4,000
NGC 224 = M31 The Andromeda    
Sb
2.3 Million
30 Million
NGC 221 = M32 Satellite of M31 with unusually high star density near core  
E2
2.3 Million
3 Million
NGC 1068 Active Galactic Nucleus  
Sbc
43 Million
  
NGC 3034 = M82, not the core black hole; mid-mass about 600 light years from core
 
Stellar
500
NGC 3115 Black hole mass measured by motion of stars  
SO
27 Million
2 Million
NGC 3377 In Leo Spur galactic group  
E5
32 Million
100 Million
NGC 3379 = M105 In Leo Spur galactic group  
E1
32 Million
50 Million
NGC 3862 = 3C264 Bright radio and x-ray source, and also has an optical jet 750 light-years long  
Elliptical
260 Million
  
NGC 4151 Active Galactic Nucleus, high-speed jets and accretion disk  
Spiral
    
NGC 4258 Deduced by maser detection and has thin, warped accretion disk    
Sbc
24 Million
40 Million
NGC 4261 Accrection disk 800 light-years across and 20 light-years off-center from host and slightly off-center from it's own black hole, and radio jets 88,000 light-years long  
,
E2
90 Million
400 Million
NGC 4374 = M84 Deduced from rotational speed of accretion disk  
Elliptical
50 Million
300 Million
NGC 4395 Active Galactic Nucleus, used to test (and, in this particular case, disprove) the starburst theory of AGNs  
Spiral
8 Million
  
NGC 4438 Black hole blowing huge bubbles of gas  
     
NGC 4486 = M87 Virgo A has bright optical and radio jet 5,000 light-years long and central accretion disk moving 1.2 million mph (550 km/s)  
, ,
E0
50 Million
3 Billion
NGC 4486b Satellite of M87    
E0
50 Million
500 Million
NGC 4594 = M104 The Sombrero    
Sa
30 Million
1 Billion
NGC 5128 Centaurus A is an Active Galactic Nucleus with high-speed radio/x-ray jet and an accretion disk tilted off-axis from black hole  
Elliptical
10 Million
1 Billion
NGC 5194-5 = M51 The Whirlpool has dark "x" of dust rings across nucleus and almost perpendicular to galactic disk  
Spiral
20 Million
  
NGC 5728 Active Galactic Nucleus has two cone-shaped beams  
Barred Spiral
    
NGC 6240 Two bright nuclei determined to be two black holes about 3000 light years apart
   
400 Million
  
NGC 6251 Active Galactic Nucleus with warped accretion disk and 3 million light-year-long jet  
 
300 Million
  
NGC 7052 Jets askew from 3,700-light-year-wide accretion disk  
Elliptical
191 Million
300 Million
PKS 0521-36 Optical jets  
     

Table Footnotes
1: In units of light years (5.8 trillion miles or 9.3 trillion km)
 
2: In units of one solar mass (4.38 x 1030 lbs or 1.99 x 1030 kg).
TABLE SOURCES: Information for most of this table done by team members Douglas Richstone (team leader), Karl Gebhardt (University of Michigan), Scott Tremaine and John Magorrian (University of Toronto, Canadian Institute for Advanced Research), John Kormendy (University of Hawaii), Tod Lauer (National Optical Astronomy Observatories), Alan Dressler (Carnegie Observatories), Sandra Faber (University of California), Ralf Bender (Ludwig Maximilian University, Munich), Ed Ajhar (National Optical Astronomy Observatories), and Carl Grillmair (Jet Propulsion Laboratory); as well as Roeland P. van der Marel (STScI), Frank van der Bosch (University of Washington), and NASA ... the rest I picked up from press releases at the Hubble Space Telescope site.

 

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